Heat sinks are used in a variety of electronic devices to dissipate heat from the device, preventing any failure of or damage to the device. Heat sinks are made of a variety of heat conducting materials and are formed in a variety of shapes to adequately conduct the heat from the device. More specifically, heat sinks, also commonly called lids, are used with integrated circuits to reduce the heat of the semiconductor die to enable the integrated circuit to function properly. The purpose of the lid is to keep a semiconductor junction, such as a junction of the transistor, below a maximum specified operating temperature. Another purpose of the lid may be to provide some physical protection for the components within the lid. A lid for an integrated circuit, commonly made of a metallic or ceramic material, is attached on top of a flip chip package to provide protection to the silicon die and other discrete surface mount components and to increase thermal conductivity of the package.
However, these types of lid heat sinks often exhibit the problem of separating from the top of an integrated circuit package. Because the adhesion strength of the adhesive used to attach the conductive lid is sometimes not strong enough to hold the conductive lid to the package, the conductive lid can come off of the package. Proper material construction and processing of the lid, the package, and the adhesive is required so that the lid will stay attached to the package during the assembly and long term field usage. Degradation of adhesion can be caused by many factors, including improper dispensing of adhesive that does not produce full adhesive surface as designed, shear stresses induced due to thermal expansion/shrinkage differences at the interfaces, push/pull stresses induced due to warping of the silicon, package, and/or board, adhesive degradation due to chemical attack, etc. Excessive component handling stresses, such as excessive torque or pressure during test, assembly, or rework, can also result in separation of the lid from the package.
Currently, a wide variety of components, such as FPGA, ASIC, graphics, microprocessors, etc., employ flipchip ball grid array (BGA) packages. As the requirements for large size silicon chips and/or passive devices used on packages increase, a smaller bond area is used to bond the lid to the substrate. However, the smaller bond area even further reduces adhesion of the lid to the substrate, often leading to lid delamination when subjected to post assembly thermal and mechanical stresses. Also, conventional designs having smaller bond areas can lead to the misalignment of the lid with respect to the substrate during lid assembly. Finally, conventional lid designs can have gaps in the epoxy bead, permitting moisture, solvents and cleaning solutions to enter the package. Such gaps can result in contamination of the thermal grease that conducts heat from the back of the die, possibly impacting the functionality of the device.
Accordingly, there is a need for an improved lid for an integrated circuit and method of securing a lid to an integrated circuit.